Point-of-sale (POS) and automated teller machine (ATM) terminals are often used at outdoor events and venues where the no-moving-parts feature of capacitive sensing is a value added feature. In conventional terminals with touch-sensitive keys, the keys are scanned in a fixed sequence. An identity thief may attempt to intercept a user's personal identification number (PIN) or password using a snooping device. Scanning keys in sequence makes it easy to correlate capacitive changes due to fingers to the physical location of the touch location.
Such conventional terminals may not include anti-snooping security features. An identity thief could log the waveforms generated by sensor array using an e-field sensor, a capacitive probe, soldering wires directly to sensor elements, or with a transparent Indium Tin Oxide (ITO) overlay. By close inspection of the scanning signature, the location of a finger can be deduced by correlating the changes in the waveform to the known key scanning sequence. It may also be possible to capture a scanning signature using a sensitive RF receiver, with the same result in loss of security. Additionally, a snooping device may scan individual sensor elements and, thus no correlation of scanning order would be required. Accordingly, in a POS application, anti-snooping features are desirable to prevent identity theft.